1. Field of the Invention
The present invention relates to an electron beam apparatus, such as a scanning electron microscope, and to a method of operating the apparatus.
2. Description of Related Art
In a scanning electron microscope, a primary electron beam that is accelerated and released from an electron beam source, such as an electron gun, is brought to a focus by the lens action of an objective lens such that the diameter of the probe is reduced on a specimen surface. Under this condition, the beam is scanned in two dimensions across the specimen surface. As a result, electrons, such as secondary electrons and backscattered electrons, are produced from the specimen and detected as a signal.
A detector for detecting such electrons is located below or above the objective lens. The output signal from the detector representative of the detected electrons is amplified as an image signal and sent to an image-processing circuit. The image-processing circuit creates image data based on the signal. The image data is sent to a display having a display unit. The display displays a scanned image based on the image data.
In an apparatus where the detector is located above the objective lens, the detector is rarely placed on the axis of the primary electron beam. Usually, the detector is placed off the axis of the primary electron beam (see, for example, JP-A-7-240168). Electrons produced from the specimen and moved upward are deflected toward the detector such that the electrons are detected by the detector.
In another method, electrons produced from a specimen and moved upward are made to hit a reflective plate such that secondary electrons produced from the reflective plate are detected by a detector (see, for example, JP-A-9-171791).
Where the detector is disposed on the axis of the primary electron beam, the detector is made of a semiconductor device or has a scintillator.
In an electron beam apparatus where an electron detector is located above an objective lens, if the primary electron beam is accelerated by low accelerating voltages, it has been difficult to detect backscattered electrons produced from the specimen in response to irradiation by the primary electron beam. Furthermore, it has been difficult to detect both backscattered electrons and secondary electrons produced from the specimen.